The invention relates to impact hammer mills having impact hammers arranged symmetrically about the periphery of a rotor on pivot shafts.
Hammers in impact hammer mills have short lives, particularly under heavy wear. Increasing wear of the hammer head in known mills reduces efficiency of comminution of materials. The impact effect of impact hammers is greatest when the impact surfaces of the hammers encounter squarely the material to be comminuted. With increasing wear of the impact hammer, however, the material to be comminuted is frequently only grazed by the worn impact hammer head. Then the material is centrifuged from the hammer and little comminution is accomplished by the worn heads.
The replacement of impact hammers in the previously known impact hammer mills requires relatively great effort and expense. The worn hammer together with its arm portion must be dismounted from the impact hammer mill and the replacement part installed. Such exchange of the entire impact hammer unit is very costly, particularly with impact hammer mills used in connection with tube mills or other aggregates. When the impact hammers are being repaired the other parts of the assembly line may also be interrupted.
The present invention substantially increases in a simple and effective manner the degree and efficiency of comminution and also the tool life.
On the hammer arm, an equalizing mass is arranged opposite the hammer head impact surface, viewed in the direction of rotation of the hammer and behind the common center of gravity line. With the aid of the equalizing mass arranged on the hammer arm, wearing of material from the hammer head rotates the hammer arm forward, in the direction of rotation of the rotor. The impact surface of the hammer head is thereby maintained in an angular position most favorable for comminution. Thus not only is the effectiveness of the hammer greatly improved but also the life of the hammer head is increased, since wear of the hammer head does not alter the angle of the impact surface. Then the impact hammer mill does not have to be taken out of operation so frequently to exchange worn hammer heads for new ones.
A further feature of the invention is that the hammer head and the hammer arm are so arranged with the equalization mass on the shaft, that the hammer impact surface lies a slight distance behind a parallel radial line of the rotor, at about 11.degree. from a radial line of the rotor through the pivot shaft and center of gravity of the hammer assembly. This arrangement of the impact surface further improves the comminution effect of the impact hammer, since the impact surface will strike squarely onto the material to be comminuted.
In one embodiment of the invention, the hammer head has a recess which is U-shaped in cross-section, with a nose inside the U-shaped recess. The upper end of the hammer arm engages this U-shaped recess and is connected thereto by a retention device. The hammer head may be mounted from above or in front on the hammer arm and is positively connected thereto. Disassembly of the entire impact hammer assembly from the rotor is not necessary.
With this embodiment and arrangement of the hammer head on the hammer arm, the hammer head has inner surfaces of the U-shape engaged with corresponding countersurfaces of the arm to be held by centrifugal forces. This insures a reliable hold on the hammer head by the hammer arm. A simple stop device, consisting only of a bolt and a spring-clamp, secures the hammer head against unintentional release from the hammer arm upon starting and stopping. The bolt and the spring do not need to withstand the centrifugal force of the hammer head and are protected within the hammer arm from the adverse environment of the hammer mill.